1. Field of the Invention
The present invention relates to a receiving apparatus, a receiving method, and a program. More particularly, the invention relates to a receiving apparatus, a receiving method, and a program whereby the manner in which to synchronize OFDM (Orthogonal Frequency Division Multiplexing) symbols is switched according to the circumstances.
2. Description of the Related Art
One modulation technique in use today for terrestrial digital broadcasting is called OFDM. According to the OFDM technique, numerous orthogonal subcarriers are provided within the transmission frequency band. Data is assigned to the amplitude and phase of each subcarrier and is digitally modulated through PSK (Phase Shift Keying) or QAM (Quadrature Amplitude Modulation).
The OFDM technique involves dividing the entire transmission band by a large number of subcarriers. That means bandwidth is limited and transmission speed is lowered per subcarrier but that the total transmission speed remains the same as that of traditional modulation techniques.
According to the OFDM technique, data is assigned to a plurality of subcarriers so that the data is modulated by performing IFFT (Inverse Fast Fourier Transform) operations. An OFDM signal resulting from the modulation is demodulated by executing FFT (Fast Fourier Transform) operations.
It follows that the apparatus for transmitting the OFDM signal may be composed of IFFT-related circuits and that the apparatus for receiving the OFDM signal may be formed by FFT-related circuits.
Given the above-outlined features, the OFDM technique has been applied frequently to terrestrial digital broadcasting setups highly vulnerable to multipath interference. The terrestrial digital broadcasting standards that have adopted the OFDM technique include DVB-T (Digital Video Broadcasting-Terrestrial), ISDB-T (Integrated Services Digital Broadcasting-Terrestrial) and ISDB-TSB.
FIG. 1 shows OFDM symbols. According to the OFDM technique, signal transmission takes place in units called OFDM symbols. As shown in FIG. 1, one OFDM symbol is constituted by an effective symbol representing a signal interval during which IFFT is carried out upon transmission, and by a guard interval (called the GI hereunder) to which a partial waveform towards the end of the effective symbol is copied.
A GI is inserted chronologically in front of the effective symbol. According to the OFDM technique, inserting the GI makes it possible to prevent interference which can occur between OFDM symbols in a multipath environment.
A plurality of such OFDM symbols are put together to form a single OFDM transmission frame. Illustratively, one OFDM transmission frame is formed by 204 OFDM symbols according to the ISDB-T standard. The position in which to insert a pilot signal is determined in units of the OFDM transmission frame.
The OFDM technique involves the use of QAM-based methods for modulating subcarriers. It follows the OFDM technique is susceptible to adverse effects such as multipath interference during transmission, with the result that the amplitude and phase of each subcarrier can turn out to be different upon receipt from what they were at the outset. Multipath interference may be caused illustratively by reflections from mountains and buildings or by SFN (single frequency network).
On the receiving side, it is thus necessary to equalize signals in order to make sure that the amplitude and phase of the received signal become the same as those initially transmitted.
According to the OFDM technique, the transmitting side inserts a pilot signal discretely in transmitted signals, the pilot signal being a known signal having a predetermined amplitude and a predetermined phase. The receiving side obtains the frequency characteristic of the transmission channel in use based on the amplitude and phase of the pilot signal so as to equalize the received signal. The pilot signal used in this manner to calculate the transmission channel characteristic is known as the scattered pilot signal (called the SP signal hereunder).
FIG. 2 is a schematic view showing a typical layout pattern of SP signals within OFDM symbols according to the ISDB-T standard. In FIG. 2, the horizontal axis represents subcarrier numbers identifying the subcarriers of an OFDM signal, and the vertical axis denotes OFDM symbol numbers identifying the OFDM symbols of the OFDM signal. The subcarrier numbers correspond to frequencies and the OFDM symbol numbers correspond to time.
In FIG. 2, each hollow circle represents data of the symbol transmitted by each subcarrier, and each solid circle denotes an SP signal. As shown in FIG. 2, an SP signal is placed at intervals of four OFDM symbols in the time direction and at intervals of 12 subcarriers in the frequency direction.
In the case of the ISDB-T standard, a signal called TMCC/AC is placed in each subcarrier. The TMCC/AC signal is designed specifically to transmit a synchronization signal that allows the receiving apparatus to know transmission parameters in effect upon transmission (i.e., information such as the modulation method in use and the encoding ratio in effect) and the symbol number of the symbol in question within an OFDM transmission frame.
In the case of the DVB-T standard, a signal called a TPS signal is inserted. As with the TMCC/AC signal according to the ISDB-T standard, the TPS signal is also formed by a synchronization signal that provides transmission parameters and permits frame synchronization. In this connection, the reader is asked to reference Japanese Patent Laid-Open No. 2005-303440.